The present invention relates to an electric motor structure and particularly to an electric motor structure providing optimum ventilation for a reversing motor used in a clothes washer application.
The present invention combines a ventilation structure for the motor with an internal fan to move air from within the motor to the environment external to the motor. The invention is particularly directed to reversing electric motors used in clothes washer applications. Historically, a number of different types of motors have been used for such applications. Although the invention is described with respect to a particular application involving a reversing, permanent split-capacitor motor, it will be understood by those skilled in the art that other types of motor applications can take advantage of the ventilation structure of the present invention.
A discussion of the types of motors used in clothes washer applications where reversing operation is required is fully discussed in copending application entitled METHOD OF DESIGNING REVERSING PSC MOTORS, Serial No. 227,146, filed Aug. 2, 1988, and assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein. Typically a problem to be addressed in reversing motors is that as a result of their frequent reversal of direction, during the course of normal operation, the motors and their accompanying ventilation means, such as fans, are not able to get up to speed after each change in direction sufficiently to remove air heated as a result of current surges through the rotor and stator windings of the motor. A motor operating in one direction normally sustains a current surge in its stator winding which is accompanied by a flux coupled current in the rotor winding when power is initially applied. Once the motor achieves steady state operation, power consumption of the motor reaches a steady state level at something less than the initial surge. The fact that the motor is turning in one direction and quickly gets up to speed, usually results in the necessary cooling being achieved by the ventilating devices provided. Such devices include openings in the motor housing to permit air flow therethrough, and internal or external fans mounted on the rotor shaft of the motor to move air, heated by the currents in the windings, away from the windings. As mentioned above, in a motor that is reversing directions frequently, there is no real steady state operation in the same sense as there is in the case of a motor turning in only one direction. Thus, the reversing motor sustains frequent current surges and heat builds up to levels considerably higher than those sustained by a motor turning in only one direction.
In order to preserve the windings in the motor, and prevent deleterious effects from the heat generated as a result of the reversing operation, special design techniques are demanded. Specifically, unique means for assembling the motor structure to maximize heat transfer away from the core of the motor, including its windings, and means for pushing or pulling air through the motor structure to enhance the cooling; for example, by creating thermal differentials between the internal and external environments, are necessary. It should also be noted, that in clothes washer applications of motors, there are typically space limitations which frequently dictate the physical size of the motor. That is, application space limitations often restrict the physical size of the motor or prevent the use of external fans, because these expedients increase the demand for space in the motor mounting area of the washing machine. Also, although the physical size of the motor and the selection of materials with which to fabricate the heat conducting elements of the motor housing are variables that typically permit some latitude in the motor design, where space limitations are not critical; where space limitations are present, and where weight is a consideration, it is important to have a motor design that can be lighter weight and made with less expensive materials which are still competent for heat transfer purposes when incorporated in an appropriate ventilation configuration. The present invention provides the combination of structure, materials, and elements to achieve the desired results.
In the present invention, two metal endshields are an integral part of the motor housing assembly. In addition, one of the endshields includes an integral mounting bracket means which contributes, along with the mass of the opposing endshield, to the heat transfer structure of the motor. These elements, of course, are capable of picking up heat transferred from the rotor and stator windings directly, and by way of the surrounding air within the endshields; the heat transfer being both by conduction and convection. By conduction, the endshields pick up heat from the stator laminations. By convection, the endshields pick up heat from the environment immediately adjacent to the windings and the stator laminations. In the case of the endshield with a mounting bracket for installing the motor to the structure of the washing machine, the heat transfer is by conduction through adjoining structure, and by convection to the surrounding air. The other endshield, of course, communicates heat to the surrounding environment by convection. Apertures located axially in the end walls of both endshields, and radial apertures in the peripheral walls of both endshields, enhance the communication of heated air from inside the motor to the external environment. A unique fan is located within the endshield having the integral mounting bracket, to further enhance the communication of heated air from within the motor structure to the external environment. Within the boundaries of the endshield in which it resides, this fan has blades which extend around the outer periphery of the stator winding, and additionally extend down towards the adjacent end of the rotor winding within the stator assembly, to break the thermal boundary layer of heated air which exists in the rotor cavity within the stator assembly. It is this thermal boundary layer which, in effect, remains mostly intact in the case of a motor that is reversing directions frequently, as is the case in a clothes washer application. Thus, the heated air that remains in close association within the stator winding cavity adjacent to the end of the rotor, is maintained in a high temperature condition, unless something is done to disrupt it. The fan of the present invention reaches down and around the top and sides of the stator assembly windings and towards the top of the rotor assembly, in close proximity therewith, to disrupt the hot air build up within the cavity and the adjacent windings. Not only does the fan disrupt this otherwise mostly undisturbed hot air buildup, but it causes the heated air to be moved so that it flows axially and radially out of the endshield surrounding that particular end of the motor. All of this is accomplished within a confined space, in what is normally anticipated to be a warm ambient applications environment.